Aluminum coordination number

Aluminum hydroxide and aluminum chloride do not ionize appreciably in solution but behave in some respects as covalent compounds. The aluminum ion has a coordination number of six and in solution binds six molecules of water existing as [Al(H20)g]. On addition of a base, substitution of the hydroxyl ion for the water molecule proceeds until the normal hydroxide results and precipitation is observed. Dehydration is essentially complete at pH 7. 

Fig. 22-2. An octahedral complex aluminum with coordination number 6.

Fig. 22-3. A tetrahedral complex aluminum with coordination number 4.

The effect of small valence and large coordination number is further shown by the observation that silicon tetrahedra, which share comers only with aluminum octahedra, share edges with magnesium octahedra (in olivine, chondrodite, humite, clinohumite) and with zirconium polyhedra with coordination number eight (in zircon). 

Other alkali ions (except lithium) also probably have the coordination Number 8 as a rule, and should similarly have a tendency to a 1 1 ratio with aluminum this is shown in NaAlSi3Os, albite, H2Na2Al2Si30i2, natro-lite, H2Cs4Al4Si9027, pollucite, etc. 

By replacing one-fourth of the silicon ions in a pyrophillite layer by aluminum ions, which can have the coordination number 4 as well as 6, the layers become negatively charged. To regain neutrality further positive ions must be introduced, such as K+ or Na+. There is room for these ions between the layers, in the pockets formed by six oxygen ions on the top of one layer and six on the bottom of the layer above. The... 

Like the three-coordinate aluminum and gallium alkoxides, the corresponding thiolates, selenolates, and tellurolates require very bulky substituents at the chalcogen atom to ensure a low metal coordination number. In addition, the steric requirements associated with the preservation of the low coordination number at the metal are increased by the longer M—S and M—Se bond lengths. The ten examples of low-coordinate,... 

There is apparently some aggregation of ions as a result of the formation of OH bridges to give species such as [(H20)4A1(0H)2A1(H20)4]4+. In the vast majority of complexes, aluminum has a coordination number of 6, but the number can also be 4 as in LiAlH4. 

In an NMR study of the MPV reduction of acetophenone with Al(OtV)3, Shiner and Whittaker (118,119) showed that the trimer is more reactive than the tetramer. Furthermore, the rate-determining step is alcoholysis of the mixed alkoxide, and not hydride transfer. They proposed that the ketone coordinates directly with trimer or tetramer by expansion of die coordination number of aluminum, and not with monomeric aluminum alkoxide. 

It is not known yet, either, if water is retained to provide for a coordination number of six of the aluminum ions. 

Monomeric dialkylaluminum hydrazides with terminal hydrazido groups (A, Scheme 2) were observed only in those cases in which donor ligands such as ether molecules (28 to 30) or hydride ions (26) saturate the aluminum atoms to yield coordination numbers of four. In the absence of Lewis bases dimers are formed of the monoaluminum and monogallium monohydrazido derivatives, a trimer has never been detected by crystal structure determinations. Three different types of structures were reported ... 

Indium(III) alkoxides are readily prepared by the treatment of trihalide with NaOR.127 The chemistry of these compounds has been little studied, no doubt because they are insoluble materials in which the indium atom optimizes its coordination number by cross-linking via oxygen. The isopropoxides of aluminum and gallium react128 with In(OPr )3 to give In[M(OPr )4]3 (M = A1, Ga) which are volatile and monomeric in solution. The structure involves six-coordination at the smaller metal centre. 

As the first step of the catalyzed reaction series, the aluminum hydroxide suspended in the reaction liquid forms an aluminum complex with the acid anion, aluminum having the coordination number four in this complex. 

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